This invention relates to a percussion mechanism for a drill rod unit particularly for drilling into rock formation or providing anchor holes. The percussion mechanism includes a percussion piston which is guided in a cylinder defined within a housing and which is moved back and forth by hydraulic pressure.
The non-uniform and sometimes unpredictable composition of rock formations or layers to be drilled often leads to difficulties in case of plain rotary drilling. Particularly when working with highly abrading materials, the drilling head carried at the end of the drill rod unit is exposed to increased wear which may result in a significant reduction of the drilling rate. Thus, the strata to be worked on may be often drilled through in a economical manner only by imparting--at least periodically for predetermined lengths of time--blows by a percussion mechanism.
German Patent No. 3,503,893 discloses a drilling apparatus for a dual head drilling wherein the drilling rod unit is formed of a tubular outer rod and an inner rod extending within the outer rod. While the outer rod is connected solely to a rotary drive, the inner rod is provided with a percussion mechanism.
The known drilling apparatus may be operated such that the outer rod--contrary to the inner rod which is also exposed to the action of the percussion mechanism--performs solely a rotary motion, or the blows imparted on the inner rod are also transmitted to the outer rod by an annular collar. Further, it is feasible to disconnect the percussion mechanism to thus work exclusively by rotating the outer and inner rods.
In the above-outlined arrangement, dependent on the operating condition, at times a significant jacket friction occurs or resistances are encountered (for example, caused by material falling into the bore hole behind the drilling head) which render drilling by the outer rod or its withdrawal difficult or even impossible.
To support the withdrawing (pulling) step, German Offenlegungsschrift (application published without examination) 29 18 631 discloses the provision of a percussion mechanism with an additional piston whose purpose is to maintain a continuous contact between the counterface of the tool and the percussion piston. In any event, in a percussion mechanism constructed in this manner--which is a component of the drilling apparatus--the resistances encountered during the withdrawing process may not always be overcome.